In computer programming, an application programming interface (API) is a set of routines, protocols, and tools for building software applications. An API expresses a software component in terms of its operations, inputs, outputs, and underlying types. An API defines functionalities that are independent of their respective implementations, which allows definitions and implementations to vary without compromising the interface. A good API makes it easier to develop a program by providing all the building blocks. A programmer then puts the blocks together.

In addition to accessing databases or computer hardware, such as hard disk drives or video cards, an API can ease the work of programming GUI components. For example, an API can facilitate integration of new features into existing applications (a so-called "plug-in API"). An API can also assist otherwise distinct applications with sharing data, which can help to integrate and enhance the functionalities of the applications.

In most procedural languages, an API specifies a set of functions or routines that accomplish a specific task or are allowed to interact with a specific software component. This specification is presented in a human readable format in paper books or in electronic formats like eBooks or as man pages. For example, the math API on Unix systems is a specification on how to use the mathematical functions included in the math library. Among these functions there is a function, named sqrt(), that can be used to compute the square root of a given number.

The Unix command man 3 sqrt presents the signature of the function sqrt in the form:

This description means that sqrt() function returns the square root of a positive floating point number (single or double precision), as another floating point number.

Hence the API in this case can be interpreted as the collection of the include files used by a program, written in the C language, to reference that library function, and its human readable description provided by the man pages.

Similarly, other languages have procedural libraries; for example, Perl has dedicated APIs for the same mathematical task with built-in documentation available, which is accessible using the perldoc utility:

In its simplest form, an object API is a description of how objects work in a given object-oriented language – usually it is expressed as a set of classes with an associated list of class methods.

For example, in the Java language, if the class Scanner is to be used (a class that reads input from the user in text-based programs), it is required to import the java.util.Scanner library, so objects of type Scanner can be used by invoking some of the class' methods:

importjava.util.Scanner;publicclassTest{publicstaticvoidmain(String[]args){System.out.println("Enter your name:");ScannerinputScanner=newScanner(System.in);Stringname=inputScanner.nextLine();System.out.println("Your name is "+name+".");inputScanner.close();}}

In the example above, methods nextLine() and close() are part of the API for the Scanner class, and hence are described in the documentation for that API, e.g.:

public String nextLine()

Advances this scanner past the current line and returns the skipped input...

Returns:

the line that was skipped

Throws:

NoSuchElementException - if no line found

IllegalStateException - if this scanner is closed

More generally, in object-oriented languages, an API usually includes a description of a set of class definitions, with a set of behaviors associated with those classes. This abstract concept is associated with the real functionality exposed, or made available, by the classes that are implemented in terms of class methods (or more generally by all its public components hence all public methods, but also possibly including any internal entity made public like: fields, constants, nested objects, enums, etc.).

The API in this case can be conceived of as the totality of all the methods publicly exposed by the classes (usually called the class interface). This means that the API prescribes the methods by which one interacts with/handles the objects derived from the class definitions.

More generally, one can see the API as the collection of all the kinds of objects one can derive from the class definitions, and their associated possible behaviors. Again: the use is mediated by the public methods, but in this interpretation, the methods are seen as a technical detail of how the behavior is implemented.

For instance: a class representing a Stack can simply expose publicly two methods push() (to add a new item to the stack), and pop() (to extract the last item, ideally placed on top of the stack).

In this case the API can be interpreted as the two methods pop() and push(), or, more generally, as the idea that one can use an item of type Stack that implements the behavior of a stack: a pile exposing its top to add/remove elements. The second interpretation appears more appropriate in the spirit of object orientation.

This concept can be carried to the point where a class interface in an API has no methods at all, but only behaviors associated with it. For instance, the Java and Lisp language APIs include the interface named Serializable, which is a marker interface that requires each class implementing it to behave in a serialized fashion. This does not require implementation of a public method, but rather requires any class that implements this interface to be based on a representation that can be saved (serialized) at any time.[a]

Similarly the behavior of an object in a concurrent (multi-threaded) environment is not necessarily determined by specific methods, belonging to the interface implemented, but still belongs to the API for that Class of objects, and should be described in the documentation. [4]

In this sense, in object-oriented languages, the API defines a set of object behaviors, possibly mediated by a set of class methods.

In such languages, the API is still distributed as a library. For example, the Java language libraries include a set of APIs that are provided in the form of the JDK used by the developers to build new Java programs. The JDK includes the documentation of the API in JavaDoc notation.

The quality of the documentation associated with an API is often a factor determining its success in terms of ease of use.

An API is usually related to a software library: the API describes and prescribes the expected behavior while the library is an actual implementation of this set of rules. A single API can have multiple implementation (or none, being abstract) in the form of different libraries that share the same programming interface.

An API can also be related to a software framework: a framework can be based on several libraries implementing several APIs, but unlike the normal use of an API, the access to the behavior built into the framework is mediated by extending its content with new classes plugged into the framework itself. Moreover the overall program flow of control can be out of the control of the caller, and in the hands of the framework via inversion of control or a similar mechanism.[5][6]

When an API implements a protocol it can be based on proxy methods for remote invocations that underneath rely on the communication protocol. The role of the API can be exactly to hide the detail of the transport protocol. E.g.: RMI is an API that implements the JRMP protocol or the IIOP as RMI-IIOP.

Protocols are usually shared between different technologies (system based on given computer programming languages in a given operating system) and usually allow the different technologies to exchange information, acting as an abstraction/mediation level between the two different environments. Protocol hence can be considered remote APIs, local APIs instead are usually specific to a given technology: hence an API for a given language cannot be used in other languages, unless the function calls are wrapped with specific adaptation libraries.

To enable the exchange of information among systems that use different technologies, when an API implements a protocol, it can prescribe a language-neutral message format: e.g. SOAP uses XML as a general container for the messages to be exchanged, similarly REST API can use both XML and JSON.

An object API can prescribe a specific object exchange format that a program can use locally within an application, while an object exchange protocol can define a way to transfer the same kind of information in a message sent to a remote system.

When a message is exchanged via a protocol between two different platforms using objects on both sides, the object in a programming language can be transformed (marshalled and unmarshalled[7]) in an object in a remote and different language: so, e.g., a program written in Java invokes a service via SOAP or IIOP written in C# both programs use APIs for remote invocation (each locally to the machine where they are working) to (remotely) exchange information that they both convert from/to an object in local memory.

Instead when a similar object is exchanged via an API local to a single machine the object is effectively exchanged (or a reference to it) in memory: e.g. via memory allocated by a single process, or among multiple processes using shared memory, an application server, or other sharing technologies like tuple spaces.

An object remoting API is based on a remoting protocol, such as CORBA, that allows remote object method invocation. A method call, executed locally on a proxy object, invokes the corresponding method on the remote object, using the remoting protocol, and acquires the result to be used locally as return value.[8]

When remoting is in place, a modification on the proxy object corresponds to a modification on the remote object. When only an object transfer takes place, the modification to the local copy of the object is not reflected on the original object, unless the object is sent back to the sending system.

For instance, through the bytecode representation, a program written in Groovy or Scala language can use any standard Java class and hence any Java API. This is possible thanks to the fact both Groovy and Scala have an object model that is a superset of that of the Java language; thus, any API exposed via a Java object is accessible via Groovy or Scala by an equivalent object invocation translated in bytecode.

On the other side, Groovy and Scala introduce first-class entities that are not present in Java, like closures. These entities cannot be natively represented in Java language (Java 8 introduced the concept of lambda expression); thus, to enable interoperation, a closure is encapsulated in a standard Java object. In this case the closure invocation is mediated by a method named call(), which is always present in an closure object as seen by Java, and in Java the closure does not represent a first-class entity.

The practice of publishing APIs has allowed web communities to create an open architecture for sharing content and data between communities and applications. In this way, content that is created in one place can be dynamically posted and updated in multiple locations on the web:

Content can be embedded, e.g. embedding a presentation from SlideShare on a LinkedIn profile.

Content can be dynamically posted. Sharing live comments made on Twitter with a Facebook account, for example, is enabled by their APIs.

Video content can be embedded on sites served by another host.

User information can be shared from web communities to outside applications, delivering new functionality to the web community that shares its user data via an open API. One of the best examples of this is the Facebook Application platform. Another is the Open Social platform.[12]

If content is a direct representation of the physical world (e.g., temperature at a geospatial location on earth) then an API can be considered an "Environmental Programming Interface" (EPI). EPIs are characterized by their ability to provide a means for universally sequencing events sufficient to utilize real-world data for decision making.

The POSIX standard defines an API that allows writing a wide range of common computing functions in a way such that they can operate on many different systems (Mac OS X, and various Berkeley Software Distributions (BSDs) implement this interface). However, using this requires re-compiling for each platform. A compatible API, on the other hand, allows compiled object code to function with no changes to the system that implements that API. This is beneficial to both software providers (where they may distribute existing software on new systems without producing and →distributing upgrades) and users (where they may install older software on their new systems without purchasing upgrades), although this generally requires that various software libraries implement the necessary APIs as well.

Microsoft has shown a strong commitment to a backward compatible API, particularly within their Windows API (Win32) library, such that older applications may run on newer versions of Windows using an executable-specific setting called "Compatibility Mode".[13]

Among Unix-like operating systems, there are many related but incompatible operating systems running on a common hardware platform (particularly Intel 80386-compatible systems). There have been several attempts to standardize the API such that software vendors may distribute one binary application for all these systems; however, to date, none of these has met with much success. The Linux Standard Base is attempting to do this for the Linux platform, while many of the BSD Unixes, such as FreeBSD, NetBSD, and OpenBSD, implement various levels of API compatibility for both backward compatibility (allowing programs written for older versions to run on newer distributions of the system) and cross-platform compatibility (allowing execution of foreign code without recompiling).

Several principles are commonly used to govern the process of designing APIs. Parnas proposed the concept of information hiding in 1972. The principle of information hiding is that one may divide software into modules, each of which has a specified interface. The interfaces hide the implementation details of the modules so that users of modules need not understand the complexities inside the modules. These interfaces are APIs, and as a result, APIs should expose only those module details that clients must know to use modules effectively. Software architecture is dedicated to creating and maintaining high-level software structures—which typically includes modules. APIs reflect interfaces between modules. Thus, a system architecture is inextricably related to the APIs that express that architecture. However, many decisions involved in creating APIs are not architectural, such as naming conventions and many details on how interfaces are structured.

These details of how interfaces are structured, as well as the software architecture, have significant impacts on software quality. For example, Cataldo et al. found that bugginess is correlated with logical and data dependencies in software.[14] This implies that to reduce bug rates, software developers should carefully consider API dependencies.

Conway's Law states that the structure of a system inevitably reflects the structure of the organization that created it. This suggests that to understand how APIs are designed in the real world, one must also understand the structures of software engineering organizations. Likewise, an API group should structure itself according to what the API needs. In a study of 775 Microsoft software engineers, Begel et al. found that in addition to coordinating regarding API design, software engineers even more commonly coordinate regarding schedules and features.[15] This reinforces the view that software organizations collaborate extensively and that organizational structure is important.

Several authors have created recommendations for how to design APIs, such as Joshua Bloch[16] and Michi Henning.[17] However, since one of the principles of API design is that an API should be consistent with other APIs already in use in the system, the details of API design are somewhat language- and system-dependent.

Protecting information on APIs from the general public. For example, Sony used to make its official PlayStation 2 API available only to licensed PlayStation developers. This enabled Sony to control who wrote PlayStation 2 games. This gives companies quality control privileges and can provide them with potential licensing revenue streams.

An API can be developed for a restricted group of users, or it can be released to the public.

An important factor when an API becomes public is its interface stability. Changes by a developer to a part of it—for example adding new parameters to a function call—could break compatibility with clients that depend on that API.

When parts of a publicly presented API are subject to change and thus not stable, such parts of a particular API should be explicitly documented as unstable. For example, in the Google Guava library the parts that are considered unstable, and that might change in a near future, are marked with the Java annotation@Beta.[18]

A public API can sometimes declare parts of itself as deprecated. This usually means that such part of an API should be considered candidated for being removed, or modified in a backward incompatible way.

When adopting a third-party public API, developers should consider the deprecation policy used by the producer of that API; if a developer publicly releases a solution based on an API that becomes deprecated, he/she might be unable to guarantee the provided service.

A narrative description of the different parts of the API and how they interact. Major frameworks in the API, such as its GUI, network, and file system frameworks should have their own separate section.

Tutorials/training classes

Step-by-step instructions that show developers how to accomplish a particular task. The text should include code that developers can copy into their own applications. For example, a training class for a cryptographic API would include code that shows developers how to use the API to encrypt a file.

the signature of the class methods including information similar to that for functions in procedural languages, possibly including a list of getter and setter methods used to access or modify encapsulated information

An API in a language using exception handling should report any kind of exception possibly thrown and the specific condition that can cause them to happen.

An API that can be used in a concurrent environment should include indications on how its behavior changes due to possible concurrent access to it: general usability in a concurrent context and possible race conditions.[4]

If the communication can be embedded in an extended conversation, and consequently how to handle the conversation

A graphical API should document:

Graphical elements it can handle

How to render graphical elements

How to lay out elements on the graphical canvas, and how to compose them

How to interact with graphical elements

How to handle user input, e.g.,

How to add callback to specific user events

How to read information from input fields

An API that interacts with a device should document how to:

Access the device to extract data from it

Modify the state of the device, when possible

Detect error conditions in the device.

An API should always indicate, where applicable:

Language version number

Library and other resource dependencies

Protocol versions it is compatible with or that it implements

Operating system or platform version it supports

An API that can be used in multiple languages via some form of language inter-operation should document any restrictions to its use by languages other than its native language.

API documentation can be enriched with metadata information: like Java annotation, or CLI metadata. This metadata can be used by the compiler, tools, and by the run-time environment to implement custom behaviors or custom handling.

In 2010, OraclesuedGoogle for having distributed a new implementation of Java embedded in the Android operating system.[20] Google had not acquired any permission to reproduce the Java API, although a similar permission had been given to the OpenJDK project. Judge William Alsup ruled in the Oracle v. Google case that APIs cannot be copyrighted in the U.S, and that a victory for Oracle would have widely expanded copyright protection and allowed the copyrighting of simple software commands:

To accept Oracle's claim would be to allow anyone to copyright one version of code to carry out a system of commands and thereby bar all others from writing their own different versions to carry out all or part of the same commands.[21][22]

In 2014, however, Alsup's ruling was overturned on appeal, though the question of whether such use of APIs constitutes fair use was left unresolved.[23]

2013 saw the creation of the "API Commons" initiative.[24] API Commons is a common place to publish and share your own API specifications and data models in any format such as Swagger, API Blueprint or RAML, as well as to explore and discover the API designs of others. The API specifications and data models declared in API Commons are available publicly under the Creative Commons license.

APIs that are intended to be used by more than one high-level programming language often provide, or are augmented with, facilities to automatically map the API to features (syntactic or semantic) that are more natural in those languages. This is known as language binding, and is itself an API. The aim is to encapsulate most of the required functionality of the API, leaving a "thin" layer appropriate to each language.

Below are listed some interface generator tools that bind languages to APIs at compile time: